Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. The incorporation of a crystalline zeolite into a matrix for catalytic cracking is known and such disclosure appears in one or more of the above-identified U.S. patents.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufacturers have equipped the exhaust systems of their vehicles with catalytic converters. These converters contain catalysts which are poisoned by tetraethyl lead. Tetraethyl lead has been widely used to boost the octane number of gasoline but may no longer be used. Refiners must now turn to alternate means to improve gasoline octane number.
One method of increasing octane number is to raise the cracking temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracking temperature also results in increased requirements for the gas plant (i.e. gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
As can well be appreciated from the foregoing, it would be extremely desirable to have a process which will provide high octane unleaded gasoline without undue sacrifice of gasoline yield. It would be even more desirable if such results could be obtained in conjunction with a marked reduction in the use of expensive additive catalysts.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having an intermediate pore size is included with a crystalline zeolite having a large pore size, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202.
Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U.S. Pat. No. 3,758,403. In said patent, the cracking catalyst was comprised of a large pore size crystalline zeolite in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 type zeolite in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite in the range of 1:10 to 3:1.
It is known that the addition of a very small amount of a medium-pore sized zeolite additive catalyst to conventional cracking catalysts results in a significant improvement in the octane number of the resultant gasoline while increasing the total yield comprised of C.sub.5.sup.+ gasoline and alkylate. U.S. Pat. No. 4,368,114 teaches this process, details its use in a fluidized catalytic cracking plant and is incorporated by reference as if set forth at length herein.
Before the advent of the present invention, it was accepted in the industry that a certain amount of catalytic activity was lost due to steaming during each catalyst regeneration. Additive catalyst addition rates were based on the rate of deactivation. The present unexpected discovery, however, will dramatically reduce the amount of catalytic activity lost during regeneration and will therefore reduce the additive catalyst makeup requirements.